The stresses both active and passive in cardiac ventricles are distributed by collagen, if the heart is analagous to skeletal muscles. This collagen will then play an important part in stress-strain relationships and determine myocardial performance via the Frank-Starling relationship. Recently, the disposition of collagen in the heart has been described using a combination of scanning and transmission electron microscopy. However, the precise role of the connective tissue must be determined using both anatomical and functional parameters, the purpose of this proposal. Data on the precise goemetry of the heart during passive pressure volume determinations and spontaneously beating hearts will be obtained by means of ultrasonic holography. This permits evaluation of the stress-strain relationship over the whole heart as well as segments permitting evaluation of inhomogeneities if present. Further, the geometric data is precise and neither spherical nor ellipsoidal assumptions need be made. This information will be correlated with connective tissue amount and distribution in rat and hamster hearts which vary in their stiffness constants by about 100%. This data will enable a more precise mathematical model of the heart to be formulated.